1. Field of the Invention
This invention relates to an image sensor device and a manufacturing method thereof, and more particularly, to a complementary metal-oxide semiconductor (CMOS) image sensor having pinned photodiode (PPD).
2. Description of the Prior Art
Complementary metal-oxide semiconductor (CMOS) image sensors are manufactured by using conventional semiconductor techniques, which have the advantages of low cost and small size. Furthermore, the CMOS image sensors have high quantum efficiency and low read-out noise. Therefore CMOS image sensor has become a prevailing image technology and replaces the charge-coupled device (CCD) over time.
A typical CMOS image sensor comprises a photodiode for sensing light. Light current from the photodiode induced by light represents a signal, whereas dark current generated from the photodiode in the absence of light represents noise. The photodiode processes signal data by using the value of the signal-to-noise ratio. Excessive dark current lowers the dynamic range of the CMOS image sensor because there is insufficient ability to distinguish between the light and dark currents. Therefore, minimizing dark current in the photodiode is a key device optimization step in CMOS image sensor fabrication.
Generally, dark current is related to surface defects, plasma damage, and wafer impurity, etc., which result from the manufacturing process. For example, after forming the photodiode of a CMOS image sensor, the surface of the photodiode tends to be damaged during the plasma etching process, and thus dark current occurs. Therefore, the prior art has provided methods to lower the occurrence of dark current. Please refer to FIG. 1, which is a schematic drawing of a conventional CMOS image sensor. As shown in FIG. 1, a CMOS image sensor 100 comprises a photodiode having a p-well 102 and an N-type heavily doped region 104. The photodiode is electrically connected to a gate 108 by an N-type lightly doped region 106, which constructs a field effect transistor with the gate 108 and another N-type lightly doped region 110. The prior art further provides a field oxide (FOX) 112, such as a local oxidation of silicon isolation (LOCOS) layer, to be a dielectric material for isolating the photodiode from other devices. The FOX 112 also covers part of the photodiode for protecting its surface from being damaged during the manufacturing processes.
Please refer to FIG. 2, which is a CMOS image sensor as disclosed in U.S. Pat. No. 6,462,365. Patent '365 provides a CMOS image sensor 200 having its photodiode 202 mostly covered by a field oxide, such as a LOCOS layer 204; and the rest of it is covered by a gate 206. Because the photodiode 202 is entirely covered by the LOCOS layer 204 and the gate 206, the surface defects resulting from manufacturing processes are prevented and thus dark current caused by the surface defects is reduced. In addition, '365 also disclosed that the LOCOS layer 204 can be replaced by a shallow trench isolation (STI).
However, the LOCOS layer formed by a LOCOS process consumes a larger surface, and occupies valuable space on a wafer when being used to isolate the photodiode from other device, which therefore reduces integration and increases cost. Comparing with the LOCOS layer, the alternative STI has more complicated processes than the LOCOS layer has, such as etching trench, filling oxidation layer, and planarization process. It is also further necessary to consider the substrate where the photodiode located may be damaged while etching the trenches.